The present invention is directed to a process for the production of molded block fuel elements for gas cooled high temperature reactors by plural step pressing of a granulated graphite material made of graphite and binder resin together with coated fissile and fertile fuel particles using a lubricant and a hydrocarbon, alcohol, phenol, amine, aldehyde, ketone or ether as an air displacing agent.
The molded block fuel elements for high temperature reactors, also called "monolith" for short, is generally a 700 to 1000 mm high hexagonal prism with a width over the flats of, for example, 360 mm and a weight of about 150 kg. The monolith is made of homogeneous fine crystalline graphite matrix of high heat conductivity, fuel regions with the same matrix and cooling channels. The fuel regions contain the fuel in the form of coated particles which are embedded in the graphite matrix. According to the construction of the fuel element the number of fuel zones usually is 138 to 216 and the corresponding number of cooling channels is 72 to 108. In contrast to a bored block and to mechanically worked graphite fuel elements with loosely filled fuel inserts the fuel zones of the monolith are well bonded to the remaining graphite matrix so that both parts of the block, that is the fuel containing graphite matrix and the fuel free zone form a monolithic structure with good heat transfer. Therewith a high cooling gas temperature is reached at relatively low fuel temperature. The further advantages of monoliths are described in Hrovat German Pat. No. 1,902,994.
The monolith is generally produced from granulated graphite material containing binder resin and coated fuel particles by molding. The principles for production is described in Hrovat German Pat. Nos. 2,104,431 and (and related Hrovat U.S. application Ser. No. 577,054 filed May 13, 1975) and 2,234,587 (and related Huschka U.S. Pat. No. 3,985,844). The entire disclosure of the Hrovat U.S. application and Huschka U.S. patent are hereby incorporated by reference and relied upon.
A series of requirements are placed on the block fuel elements. Besides high strength and conductivity properties there are demanded of the block matrix narrow tolerances in dimension. The outer dimensions of the hexagonal prism as well as the diameter and the positions of the numerous cooling channels and fuel regions produced by pressing only permit a deviation of several tenths of a millimeter together and for the longitudinal axis of the block from the nominal value. Since the coating of the fissile and fertile fuel particles must remain intact in the production of the fuel element, the pressure in molding and ejecting the block from the die is limited.
According to the previously known molding process in spite of the use of a lubricant, the block fuel elements cannot be ejected in stable form from the tool because of the unfavorable block shape and numerous metal rods installation for molding of cooling channels (72-108 per block). The block ejected in the plastic range of the binder resin have a tendency to form cracks and change shape. Below the plastic range the lubricant likewise begins to solidify whereby the friction increases so sharply that in ejecting the permissable load of the coated particles is exceeded.
The problem of the present invention therefore was to get around the described technological difficulties in the production of block fuel elements for high temperature reactors and to be able to eject the finished molded block fuel element undamaged from the tool at such a low pressure that the mechanical integrity of the coated particles is not endangered.